In a prior art, for example, in an image pickup apparatus having an image pickup device (which is called a linear-logarithmic sensor) having a linear photoelectric conversion characteristic in a low illuminance side and a logarithmic photoelectric conversion characteristic in a high illuminance side, (which is called a linear-logarithmic characteristic), there was a case that the inflection points vary in response to the characteristic variation of an element configuring the pixel of the image pickup device. And it has given more than a few bad influences on image quality.
Thus, with respect to a correction method for correcting the variation of the inflection point, a method for converting image data of all pixels obtained when the image was picked up to the data on a standard photoelectric conversion characteristic has been proposed in Japanese Patent Application Open to Public Inspection No. H11-298799. Further, Japanese Patent Application Open to Public Inspection No. H11-298799 discloses a method for memorizing the ratio between the temperature coefficients of all the pixels against the ambient temperature and the standard temperature coefficient, and a method for correcting the temperature characteristic based on the ambient temperature detected by a temperature sensor.
Japanese Patent Application Open to Public Inspection No. 2000-175108 discloses a method for correcting a signal in a linear characteristic area after correcting a signal offset in a logarithmic area so as to have an arbitrary offset and detecting the temperature of an image pickup device to conduct temperature correction.
However, according to the method disclosed in Japanese Patent Application Open to Public Inspection No. H11-298799, in order to divide the photoelectric conversion characteristics of all the image pickup device of two million pixels used in a less expensive digital camera into 1,000 (one thousand) illuminance, each area of which has a 16 bit (two bytes) data form, and to memorize the data, 4 G Byte memory, which is derived by a formula; 2 million×1,000×2, which is more than a huge memory installed in a personal computer is required. Thus, this method will not be applied to an image pickup apparatus such as a digital camera. Further, in order to compare the imaged data of all the pixels to the memorized photoelectric conversion characteristics to correct the imaged data onto the data on the standard photoelectric conversion characteristic based on the result, huge number of calculations and operation time to perform the calculations are required. Thus, this method will not be realistic to be applied to an image pickup apparatus such as a digital camera.
Further, according to the method disclosed in Japanese Patent Application Open to Public Inspection No. 2000-175108, as shown in FIG. 16(a), when considering a photoelectric conversion characteristic 601 (hereinafter, referred to as standard characteristic), which is a linear logarithmic characteristic and a standard photoelectric conversion characteristic for all the pixels and a photoelectric conversion characteristic having a shifted inflection point against the standard characteristic (hereinafter, referred to as comparative characteristic), if trying to match the comparative characteristic 603 (having a linear characteristic 601a and a logarithmic characteristic 603c in each side of a inflection point 603d) to a standard characteristic 601 (having a linear characteristic 601a and a logarithmic characteristic 601c across a inflection point 601d) by conducting a parallel shift (hereinafter, referred to as an offset) 605 in the vertical direction of the drawing, a portion, in the logarithmic characteristic 603c, where an image pickup surface illuminance is higher than Lm coincides to the logarithmic characteristic 601c. However a portion 603b, in the logarithmic characteristic 603c, where the image pickup surface illuminance is lower than Lm becomes a logarithmic characteristic 607 band does not coincide to a portion 601b, in a linear characteristic 601a, where the image pickup surface illuminance of the portion 601a is higher than Ln. Namely, inflection point variation cannot be corrected.
Here, FIG. 16(a) illustrates a schematic diagram for showing a graph of two photoelectric conversion characteristics 601 and 603 including a linear-logarithmic characteristic having inflection variation (inflection points 601d and 603d). FIG. 16(a) has a horizontal axis, which is a logarithmic axis of an image pickup surface illuminance L and a vertical axis, which is a linear axis of a photoelectric conversion output VP. The image pickup surface illuminance of the inflection point 601d is Lm and the photoelectric conversion output is Vm. The image pickup surface illuminance of the inflection point 603d is Ln and the photoelectric conversion output is Vn.
As illustrated in FIG. 16(b), the inflection point drifts in response to the temperature. According to the measurement results of an inventor of the present invention, as illustrated in FIG. 16(b), under the condition of a practical temperature range from 0° C. to 65° C., characteristic in the linear characteristic area has little changes due to the temperature change. However, in the logarithmic characteristic area, the inflection point shifts toward the lower illuminance side and the slope of the characteristic becomes steeper as the temperature rises. The reason why the slope of the characteristic becomes steeper is that the logarithmic characteristic is characteristic, which proportions to the absolute temperature.
Japanese Patent Application Open to Public Inspection No. H11-298799 discloses a method for memorizing the ratio between the temperature coefficients of all pixels against ambient temperature and the standard temperature coefficient, and correcting the temperature characteristic based on the ambient temperature detected by a temperature sensor. However, the ambient temperature detected by the temperature sensor is not always real temperature of an image pickup device. Thus the temperature cannot be correctively corrected by the method disclosed in Japanese Patent Application Open to Public Inspection No. H11-298799.
Japanese Patent Application Open to Public Inspection No. 2000-175108 also discloses a method for detecting the temperature of an image pickup device by a temperature sensor and conducting temperature correction. However, there is no explanation about the temperature sensor. Accordingly, it cannot be said that temperature measurement of the image pickup device can be conducted.